FROM The Longest Day to Saving Private Ryan, the D-Day landings cornered the market in cinematic heroics with "might and right" rolling the Nazis back to Berlin.

American soldiers march along the Champs Elysees, four days after the liberation of Paris

In true life, Europe was indeed saved by a tidal wave of courage and bravery that ensured freedom for generations but part of the horrific aftermath has remained buried by a mountain of sugar-coated Hollywood scripts.

The all-conquering GI was soon viewed with suspicion in France after US forces were accused of thousands of rapes, intimidation and having sex in daylight in parks, cemeteries and among the rubble of destroyed houses.

Controversial new research has revealed that the Normandy invasion of the Second World War was a supercharged erotic adventure for some who, after fighting their way off the murderous beaches, expected to find free love from a grateful public.

As they trained for the military operation in the UK, the GIs were motivated by lurid stories of how French women had few morals and would be swift to show their appreciation. The sexual propaganda was irresistible. Life magazine, one of the prominent publications of the day, stated: "France was a tremendous brothel inhabited by 40,000,000 hedonists who spent all their time eating, drinking and making love."

Senior officers encouraged the myth as a way of masking the obvious horrors of a daring attack that would result in 3,000 deaths in the first 24 hours.

Professor Mary Louise Roberts says: "To motivate the GIs to fight in Europe, the US military lavished the official trench newspaper, Stars And Stripes, with pictures of French women kissing American soldiers. France, soldiers were led to believe, was a land of beautiful women eager to show their gratitude for liberation." Prof Roberts' book What Soldiers Do, charts a darker side of events after the landings on June 6, 1944. Stars And Stripes increased expectation by printing useful French phrases for the GIs, which included: "You are very pretty," and "Are your parents at home?" French women were branded "sign language girls" because of rumours that they could be seduced by a simple series of hand gestures, the book reveals.

Painstaking research through US military records and municipal documents in Normandy helped Prof Roberts piece together the scandalous behaviour of some of the hundreds of thousands of US troops who were part of the campaign.

"Sexual fantasies about France did indeed motivate GIs to get off the boat and fight but such fantasies also unleashed a veritable tsunami of lust," she says.

Roberts, a history professor at the University of Wisconsin, was the first American to view many of the French archives and, although it has unleashed a firestorm of criticism, her book is being hailed as an important perspective on the conduct of war.

"I truly believe what we did there was amazing but I am interested in providing a richer and more realistic picture," she says. That tableau features a shell-shocked French public agonising over a liberation laced with a different danger. An ironic joke in Normandy at the time went: "With the Germans, the men had to camouflage themselves but when the Americans arrived, we had to hide the women." Armed with an abundance of cigarettes, food and money, the GI could buy what he wanted and prostitution became rife. Some did not want to pay and the rape statistics grew while commanders turned a blind eye.

When French outrage eventually forced action, a series of kangaroo courts resulted in 153 prosecutions for sex offences and 29 soldiers being hanged for rape, 77 per cent of them were black GIs. The military had to send for an executioner from Texas to perform the hangings as the French capital punishment system was still the guillotine and they had no technicians skilled in the use of the noose.

Former Resistance fighters in Troyes found themselves pressed back into service as vigilante squads to prevent "the bad manners and assaults which American soldiers dish out to our women and girls". Pierre Voisin has, until now, not featured in any military history but the businessman elected mayor of the strategic Channel port of Le Havre found himself in a long-running battle with US military. The port, a keystone in Hitler's Atlantic Wall, was blasted to wasteland as the Royal Navy fired 4,000 tons of shells and Allied aircraft dropped 1,900 bombs to dislodge the German garrison. It was only three months after D-Day that it was liberated and became the US logistical base through which munitions, supplies, transport and soldiers poured into Europe.

They were billeted in huge camps dotted around Le Havre which they named after such cigarette brands as Lucky Strike, Chesterfield and Philip Morris and their presence enticed hungry and desperate women from miles around to indulge in prostitution, further underscoring the view that sex was uppermost in the thoughts of French females.

GI promiscuity took place in parks, cemeteries, streets and abandoned buildings in cities

Professor Mary Louise Roberts

"GI promiscuity took place in parks, cemeteries, streets and abandoned buildings in cities," Prof Roberts says in the book. "Sexual relations became unrestricted and public; sexual intercourse was performed in daylight before the eyes of civilians, including children.

"Locals in cities like Le Havre and Reims condemned such displays as scandalous and degrading." Voisin kept up a barrage of complaints to Colonel Thomas Weed but his concerns were dismissed. The military imperative was to keep the news from the American public and stem the rising tide of venereal disease.

The priorities were clearly signposted when chief surgeon AW Kenner called for immediate action noting there could be "a dangerously high incidence of venereal disease among our troops, entailing serious loss of fighting efficiency". To the outrage of the French, GIs were issued with more condoms to solve the problem. The Germans had a different approach, establishing official brothels where prostitutes were subject to regular medical checks and a data bank was kept of their sexual health.

Le Havre became the "Wild West of France", according to one resident. Hundreds of prostitutes flocked there, many braving trips through the advancing front line to join the lucrative trade.

Unofficial brothels sprang up but, as they had to remain unmarked, GIs would roam the streets calling up at windows for sex or knocking on doors demanding "service".

Residents complained that GIs had punched two holes in a cemetery wall as an entrance and exit for prostitutes plying their trade in the graveyard. Voisin sent extra police patrols to round up prostitutes and put them on trains out of town but they simply got off at the next stop and, as they had made so much money, got taxis back to Le Havre. With hospitals unable to cope with the number of girls needing treatment for STDs, the mayor called on the US to create regulated brothels near the camps outside the town. No action was taken.

The problem was replicated across France and into Paris, where four prostitutes were murdered by GIs after the city was liberated. In Mourmelon, a "crowd of prostitutes" set up in parks and fields and the town's cemetery became a "military theatre of debauchery". Officials in nearby Chalons-sur-Marne recorded huge levels of fighting, assaults and break-ins, all with prostitution at their root. In Lison, near Caen, and in Reims, prostitutes worked in daylight near train stations and one public record states: "The children are aware of these women's love cries and their responses to such scenes are sometimes frightening."

The book powers a spotlight on a side of war that is often shadowed by the loftier ambitions of a campaign. Rape, violation and sexual humiliation have been a toxic by-product of war and even modern warfare cannot escape the taint. It should not serve to dilute the bravery that drove the Allies to victory but it is a powerful testimony that war brutalises everyone and that a duty of care extends beyond the military machine.

What Soldiers Do: Sex And The American GI In World War II France by Mary Louise Roberts is available from press.uchicago.edu